1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for down-hole communications via drill strings.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
It is known in the art to communicate through drill strings so that the drill operator at the surface might understand what is happening underground miles away at the drill bit. Stress waves are created in the drill string and data are modulated on them. These waves then use the drill string as a wave guide and carry the data up and down the well. Unfortunately, the communication range is limited by unwanted wave reflections at the joints between various components of the drilling system as well as the various components of the communication system itself. The present invention provides a way to eliminate these reflections and extend the range indefinitely. It is commonly known that wave reflections are caused by changes in the physical parameter called wave impedance. All components in a drill string and all components in a drill-string communication system have a wave impedance.
For example, the instruments and transmitters in a drill-string communication system are housed in cylindrical enclosures and the wave impedance of a uniform cylinder is equal to the algebraic product of the material density, sound speed, and cross-sectional area of the enclosure. If the individual wave impedances of these various components are matched then the wave reflections between them are eliminated. However, the impedance of a drill string is a complex number, while those of the communication devices are real numbers. In general a real number cannot be made to equal a complex number, and thus it appears that it is impossible to impedance match the communication system to the drill string. However, the present invention takes advantage of the discovery that the impedance at certain physical locations along the length of each and every piece of drill pipe in the drill string is also real. At these points, the impedance of the communication devices can be matched to the drill string.
The present invention is of a system and method for transferring wave energy into or out of a periodic structure having a characteristic wave impedance profile at a prime frequency, the characteristic wave impedance profile comprising a real portion and an imaginary portion, comprising: locating one or more energy transfer elements each having a wave impedance at the prime frequency approximately equal to the real portion of the characteristic wave impedance at one or more points on the periodic structure with the imaginary portion approximately equaling zero; and employing the one or more energy transfer elements to transfer wave energy into or out of the periodic structure. In the preferred embodiment, the periodic structure comprises a drill string comprising a plurality of sections of drill pipe of approximately equal length and a plurality of sections of tool-joint connections of approximately equal length. The one or more energy transfer elements preferably comprise one or more repeaters. Each of the one or more energy transfer elements is preferably located approximately at a longitudinal midpoint of a section of drill pipe or tool joint connection, although other points with real impedances will also work. One or more quarter-wave transformers can be provided at one or more points on the periodic structure with the imaginary portion approximately equaling zero to transmit waves across one or more discontinuities in the drill string. As one way to match the impedances, each of the one or more energy transfer elements comprises a cross-sectional area approximately equal to the real portion divided by a product of a mass density and a speed of sound of the energy transfer element. A terminator can be employed for cancellation of waves in the periodic structure. The invention substantially eliminates reflections of the wave energy at the prime frequency by joints between sections of the periodic structure. The periodic structure can not only be a drill string, but also any of optical structures, laminated slabs, semiconductor chips, coated lenses, pipes, and geologic formations.
The present invention is also of a stress wave communication system and method comprising: in a periodic structure transmitting stress waves at a communications frequency; and employing a terminator comprising an amplifier having a gain of negative one at the communications frequency. In the preferred embodiment, the periodic structure comprises a drill string comprising a plurality of sections of drill pipe and a plurality of sections of tool-joint connections. The terminator preferably employs a filter to remove frequencies higher than a predetermined frequency above the communications frequency, the filter located between an accelerometer array and a transmitter array.
The present invention is additionally of a stress wave communication system and method comprising: providing a periodic structure having a characteristic wave impedance profile at a communications frequency, the characteristic wave impedance profile comprising a real portion and an imaginary portion; and employing a quarter-wave transformer having an impedance approximately equal to a square root of a product of an impedance of a member connected to the periodic structure at the communications frequency and the real portion at a point at which the imaginary portion approximately equals zero. In the preferred embodiment, the periodic structure comprises one or more drill strings each comprising a plurality of sections of drill pipe of approximately equal length and a plurality of sections of tool-joint connections of approximately equal length. The transformer preferably has a slower wave speed than the periodic structure.
The invention is further of a stress wave communication system and method comprising: providing a periodic structure for transmitting stress waves at a communications frequency and having a characteristic wave impedance profile at the communications frequency, the characteristic wave impedance profile comprising a real portion and an imaginary portion; and employing one or more repeaters each having a wave impedance at the communications frequency approximately equal to the real portion of the characteristic wave impedance at one or more points on the periodic structure with the imaginary portion approximately equaling zero. In the preferred embodiment, the periodic structure comprises a drill string comprising a plurality of sections of drill pipe of approximately equal length and a plurality of sections of tool-joint connections of approximately equal length. Each of the one or more repeaters is preferably located approximately at a longitudinal midpoint of a section of drill pipe or tool joint connection, although other points with real impedances will also work. Each of the one or more repeaters preferably comprises a cross-sectional area approximately equal to the real portion divided by a product of a mass density and a speed of sound of the repeaters.
The invention is still further of a method of inserting wave energy into a periodic structure comprising: determining a point in a periodic structure having a characteristic wave impedance profile at a communications frequency, the characteristic wave impedance profile comprising a real portion and an imaginary portion, which point has an imaginary portion approximately equaling zero; matching impedance of a wave energy insertion elements to the real portion at said point; and inserting at the point via the wave energy insertion elements wave energy at the communications frequency; and whereby reflections of the wave energy by the periodic structure are substantially minimized. In the preferred embodiment, the periodic structure comprises any of optical structures, laminated slabs, semiconductor chips, coated lenses, pipes, geologic formations, and drill strings.
A primary object of the present invention is to provide a communications method and apparatus functional over a drill string of any length.
A primary advantage of the present invention is its extensibility to other periodic structures.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.